We study mass bounds of the W' gauge boson in generic left-right symmetric models. Assuming that the gauge bosons couple universally to quarks and leptons, we allow different gauge couplings g_R != g_L and mass mixing, V^L_{CKM} != V^R_{CKM} in the left and right sectors. Imposing constraints from collider experiments and K^0, B_d, B_s physics, we investigate scenarios where W' is lighter, or heavier than the right handed neutrino ν_R. In these scenarios, W' mass bounds can be considerably relaxed, while Z' mass bounds are much more stringent. In the case where M_{W'} \leq M_{ν_R}, the experimental constraints come from W' -> tb and W' -> jj channels, while if M_{W'} \geq M_{ν_R}, the dominant constraints come from W'->lljj. The observed (expected) limits in the two-dimensional (M_{W'},M_{ν_R}) mass plane excluded at 95% confidence level extend to approximately M_{W'}= 3.1 (3.3) TeV in the ee channel and 3.3 (3.4) TeV in the (μμ) channel, for a large range of right-handed neutrino masses up to M_{ν_R}= 2.1 (2.1) TeV in the ee channel and 2.6 (2.5) in the (μμ) channel, representing a significant relaxation of the mass bounds. We also perform a complete and consistent analysis of the alternative left-right symmetric model, emerging from E6 grand unification, including all present theoretical and experimental constraints, with a particular emphasis on dark matter observables and collider signals. We show that the exotic neutrino isodoublet in this model, the scotino, is a viable candidate for dark matter, satisfying relic density and direct detection constraints. Its mass is restricted to a narrow region, and this has implications on the Higgs bosons spectrum of the model, rendering it predictable. In particular, there must be a few light scalar, pseudoscalar and charged Higgs. We also show that the additional charged boson W' can be light, and indicate the most promising signals for this model at the high-luminosity LHC.